Ultrasonic homing beacon and communication equipment for underwater swimmers



ULTRASONIC HOMING BEACON AND COMMUNICATION EQUIPMENT FOR UNDERWATER'SWIHMERS Filed Aug. 17,. 1967 INVENTOR. FRANK M4554 JR United StatesPatent US. Cl. 340-6 Claims ABSTRACT OF THE DISCLOSURE This is a homingsystem for guiding underwater swim mers to a moored ultrasonic beaconwhich sends out omnidirectional signals. The swimmer carries anultrasonic compass comprising a compact housing that includes adirectional receiving hydrophone and a signal strength indicating meter.Since the receiver is directional, the strength of the signal which itreceives will, of course, be maximum when the receiver is pointingdirectly at the beacon. Therefore, the swimrner orients the axis of thereceiver to obtain a maximum signal reading on the meter scale. Then, heswims in the direction in which the receiving hydrophone is pointing,and this takes him to the moored ultrasonic beacon.

This invention relates to an ultrasonic homing beacon and communicationsystem which is particularly useful for guiding underwater swimmers, andmore particularly, to systems for assisting them during conditions ofpoor visibility.

A primary object of my invention is to provide an ultrasonic homingbeacon system especially well suited for guiding underwater swimmers.More particularly, an object is to provide a system utilizing (1) afixed underwater transducer capable of generating omnidirectionalultrasonic signals which acts as a guiding beacon, and 2) a compact,directional, ultrasonic compass which may be carried by an underwaterswimmer to enable him to move in the direction of the beacon. Here, afurther object is to indicate to him the approximate distance whichseparates him from that beacon.

Another object of my invention is to provide a compact, highly effcient,directional, ultrasonic receiver which has maximum sensitivity along aparticular axis. Here, an object is to provide such a receiver with apredetermined angle of sensitivity which is wide enough to make it easyto locate the beacon. Conversely, an object is to provide a receiverhaving a sensitivity which falls oif rapidly for angles which areoutside the predetermined angle of maximum sensitivity. A specificobject of the invention is to provide an ultrasonic receiver that hasdirectional characteristics which are relatively free of secondary lobeshaving any significantly high sensitivity at angles other than thedesired directional axis.

A still further object of my invention is to design a directional,ultrasonic, underwater receiver having a twopiece housing structurewhich includes a directional hydrophone structure formed by suitablyshaping the outer portion of one of the housing sections.

3,489,993 Patented J an. 13,- 1970 Yet another object of my invention isto provide an underwater system, of the described type, which includesmeans for transmitting warning signals to call underwater swimmers backto their beacon station.

In keeping with an aspect of the invention, these and other objects areaccomplished by a homing system which operates somewhat as an underwaterultrasonic direction finder operates. For present urposes, my system maybe considered a portable device which is somewhat analogons to anultrasonic compass. The ultrasonic compass is mounted in a compactwaterproof housing which may be carried in any suitable manner; forexample, it may be strapped to the divers Wrist. This ultrasonic compasscomprises a directional receiver in combination with a meter thatindicates the strength of a received beacon signal. Thus, there is amaximum deflection when the receiving axis of the compass is alignedwith the azimuth of the beacon. In order to keep the equipment small, Ihave found it desirable to use signals in the ultrasonic frequencyregion in the approximate range 40 kHz. to kHz. At lower frequencies,the physical dimensions of the receiver become too large, and at higherfrequencies, the attenuation loss in water becomes appreciable if therange exceeds a few hundred yards.

The novel features which are characteristic of this invention are setforth with particularity in the appended claims. However, the inventionitself both as to its organization and method of operation will best beunderstood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a front view of a meter for providing one type of read-outwhich tells an underwater swimmer about his position with reference to afixed beacon;

FIG. 2 is a cross-sectional view of the ultrasonic compass taken alongthe line 2-2of FIG. 1;

FIG. 3 illustrates the directional response characteristics of thehydrophone receiving element contained in the ultrasonic compass of FIG.2; and

FIG. 4 illustrates a beacon suspended from a floating buoy which isanchored to the ocean floor.

Briefly, FIGS. 1 and 2 show a portable ultrasonic compass incorporatingthe principles of the invention. Basically, this compass has three majorportions; an ultrasonic horn and transducer assembly 20, electronicequipment 21 for processing signals produced by the transducerresponsive to sonic energy picked up at 20, and a device 22 for giving aread-out responsive to the output of the electronic circuit. When theswimmer points the horn and transducer assembly 20 directly at thebeacon station, the read-out device 22 gives an indication of a maximumstrength signal. This way, the swimmer may scan with the horn 20 whilehe watches the read-out device. When he observes the maximum signalindication, he swims in the direction in which the horn is pointing, andthat takes him back to the beacon.

More particularly, a preferred housing structure including asemi-spherical or dome shaped end-cap 25 which mates with and completelycloses the open end of another cup-shaped housing structure 26. AnO-ring 27 and screws 28', 28" enable an assembly of sections 25 and 26into a completely watertight enclosure. For the embodiment here shown,the housing portion 26 may be either a molded, rigid plastic, waterproofmaterial or a metal casting or forging, as desired. The dome-shapedhousing portion 25 may be made of a transparent material such as Lucite.Or, an alternative structure may include a composite assembly of anopaque ring portion having a pressure resistant glass window suitablybonded therein. By using a frequency of approximately 60 kHz., theentire structure can be built with an overall dimension havingapproximately a 3" diameter. This is a suitable and convenient size fora device used as an underwater ultrasonic compass. However, the physicalsize may be made somewhat larger or smaller depending upon the choice offrequency and sharpness of the receiving beam that is desired. By makingthe structure in approximately the shape illustrated in FIG. 2,. theunit may be conveniently strapped to the wrist of an underwater swimmer.He may easily observe the meter scale while moving his arm to determinethe orientation of a maximum reading, thereby making an alignment uponthe homing signal from the ultrasonic beacon.

A chassis 29', 29" is attached inside a recessed area of housing portion26 by means of a plurality of screws 30, 31. A read-out means 22, hereshown as an electrical meter, is attached to the other end of thechassis 29', 29" by means of studs and nuts 32, 33. Preferably, themeter has a wide pointer 35 (FIG. 1) so that it may be seen easily whenit is under water.

At its opposite end, the housing portion 26 terminates in a recessedconical frustrum cavity. Preferably, if the lines 36, 37 were extendeduntil they meet, the angle of the sides of the conical frustrum cavitywould be approximately 90. The wall of this cavity is lined with a thinlayer of low acoustic impedance material 38, such as a mixture of rubberand cork, one example of which is sold under the trade name of Corprene.

A transducer in the form of a thin-walled polarized ceramic cylinder 39is cemented to the Corprene lined surface on the bottom of the recessedcavity For this cylinder, I prefer to use a polarized barium titanate orlead zirconate, both of which are well known transducer materials. Theinner surface and the free-end of the cylindrical ceramic transducerelement 39 are also lined with a thin layer of the low acousticimpedance material 40, 40 such as the Corprene mixture.

A potting compound, having approximately the acoustic impedance ofwater, completely fills the conical frustrum cavity 20. There are manywell known potting compounds, of which the family of epoxies aretypical, that satisfy the necessary requirement in that they arewaterproof and provide good acoustic coupling between the ceramicelement 39 and the water.

Insulated conductors 46 and 47 are used to electrically connect theelectrodes 44, 45 deposited or otherwise formed on inner and outersurfaces of the ceramic cylinder to electronic equipment 54. Theseconductors pass through and are sealed in clearance holes 48, 49 in thebase of the conical frustrum. The details of the electronic circuit 54are not shown since they are prior art circuits and are not part of thisinvention. However, it is thought that any one of many conventionalcircuits may be employed to achieve the desired signal detection andamplification. In addition, band pass filters may also be included inorder to reduce the sensitivity of the system to background noise andthereby improve the signal-to-noise ratio.

A battery 55 is schematically shown as attached to the chassis 29" by aclip 56. Any suitable electrical contacts are connected between thepositive and negative terminals of the battery and the correspondingpower input terminals of the electronic circuit 54. Since the outputmeter 22 has extremely low power requirements, the battery 55 has verylow drain, therefore, it does not have to be changed exceptinfrequently, after relatively long periods of use. The batteryreplacement may be easily accomplished by removing the dome-shaped clo-4 sure cap 25. Alternatively, a rechargeable battery may be used.

Means are provided for indicating when a maximum strength signal isreceived. In greater detail, an ultrasonic amplifier is included in box54 for increasing the strength of signals generated by the transducer39. The amplified signals may then be rectified, applied to, and read ona D.C. milliammeter 22. Since the horn 20 is directional, the magnitudeof the picked-up signals is an indication of both direction and distanceto the fixed beacon. Therefore, at a maximum meter reading, the hornpoints at the beacon and the needle 35 points to numbers indicating thedistance between the swimmer and the beacon.

The directional pattern of the receiving hydrophone is illustrated inFIG. 3. More particularly, a hydrophone which is constructed as shown inFIG. 2, has a directional receiving response which is essentially asingle lobe fanning out at a predetermined angle, here shown as about60. The response throughout the region outside the main lobe beam anglefalls off to at least 25 db below the level of sensitivity on the mainaxis of maximum response. The pattern is free of the usual secondarylobes that are associated with pistons and other directional elements.This means that no false bearings are obtained since the highsensitivity response lies only in the predetermined angle of the mainlobe. Therefore, the maximum sensitivity of this hydrophone is, along anaxis corresponding to the center line of the ceramic cylinder 39.

FIG. 4 schematically illustrates an underwater ultrasonic beacon used toprovide a fixed point of reference for the underwater swimmer. Morespecifically, the beacon utilizes a transmitting transducer 60 suspendedby a cable 61 from a buoy 62. This transmitting transducer may beenclosed within a molded rubber cover which not only protects thetransducer but also furnishes a waterproof covering which is sealed tothe jacket of the underwater cable 61. A line 63 attaches the buoy 62 toan anchor 64 which keeps the transmitting beacon at a fixed location.Instead of the anchor 64, the line 63 could, of course, be attached toeither an anchored ship or any other fixed reference point. Also, thebuoy 62 may be omitted, and the line 61 may be connected directly to aship. The power for operating the transmitting transducer 60 may befurnished in any conventional manner. For example, the power supplycould be in the buoy 62; or, it could be a portable or permanent type ofshipboard electrical generator.

The transmitting transducer 60 may consist of a piezoelectric ceramiccylinder identical to the cylinder 39 illustrated in FIG. 2. In itsradiation characteristics, the transducer 60 is omnidirectional in boththe horizontal and vertical directions provided that the length of theceramic transmitting transducer cylinder is not greater than aboutone-half of a wavelength. If the length of the ceramic cylinder is madelonger, e.g. in the order of one wavelength, the response along thevertical axis of the suspended transducer is much less; however, thesound radiation continues to be omnidirectional in the horizontal plane.As an optimum, I prefer to use a ceramic cylinder which has a lengththat is approximately three-fourths of a wavelength. This optimum sizeresults in an omnidirectional response for a large vertical anglethroughout 360 of horizontal azimuth.

In operating the direction finder, the fixed beacon represented by thetransducer 60 emits an ultrasonic signal which is transmitted outwardlyin all directions. For the simple purpose of finding the direction to amoored ultrasonic underwater beacon, the transmitted beacon signal maybe either a continuous signal or, preferably, a series of repetitivebursts of ultrasonic signals. The bursts should recur at a sufficientlyrapid repetition rate to maintain a constant meter reading when thedirectional receiver is brought into alignment with the beacon. An

underwater swimmer equipped with the ultrasonic compass, shown in FIG.2, then obtains a maximum indication on the meter 22 when he aligns thereceiving axis of the horn 20 with the ultrasonic beacon set out fromtransducer 60.

The distance between the beacon and the compass is indicated by thestrength of the signal which is received. In greater detail, theintensity of the ultrasonic signal sent out from the beacon is set to apredetermined level by the design and adjustment of the equipment. Thesensitivity of the ultrasonic amplifier in box 54 of the receivinghydrophone of FIG. 2 is adjusted so that the output meter 22 readsapproximately a full scale at a close range of about to 20 yards. Atgreater distances, the maximum reading of the output meter 22 decreasesbecause the signal level attenuates as the range increases. For example,if a full scale meter reading represents a range of approxi matelytwenty yards, the maximum meter reading ob tained at approximatelytwo-hundred yards should be about 10% of a full scale reading assumingthat attenuation through the water is negligible. The reading would besomewhat less than 10% of full scale if attenuation is taken intoaccount.

From the foregoing, it should be clear that the described systemprovides two items of information for the underwater swimmer. First, byrotating the directional receiver which he holds until the output meterreading becomes maximum, he receives the reltaive bearing of the beaconwith respect to his position. Second, he finds his approximate distancefrom the beacon by the magnitude of the meter reading when it is alignedwith the beacon.

Means are provided for improving the visibility of the meter reading.That is, visibility is improved by making the meter pointer 35relatively wide, and by marking the meter scale with the approximaterange in yards, without showing exact points on the scale. Anapproximate indication of distance is here given within an acceptableprecision; however, it is recognized that a less visible pointer andscale are more accurate.

Instead of a simple meter indicator, I may also use a sonic indicator.In this case, the ultrasonic signal which is received causes electronicequipment 54 to generate a proportional audible signal that istransmitted to the swimmer in any suitable manner, as from a secondtransducer built into the face of the housing portion 25. Or, the secondtransducer may be an ear plug 70 which is connected to the electronicequipment 54 via an underwater connector 71. In this arrangement, theintensity of the audible signal is maximum When the receiver is pointedtoward the moored beacon, and the relative volume of the signal is anaudible indication of the distance from the moored beacon.

A third alternative embodiment used lights to give the guidance andrange information to the swimmer. For example, several lights may bemounted under dome 25 in place of the meter 22. A selective number ofthese lights are turned on according to the relative intensity of thesignal that is picked up by the receiving hydrophone. Thus, when thereceiving hydrophone is in alignment with the moored beacon, the maximumsignal strength is indicated because a maximum number of lights are litfor that signal strength. For example, as an indication of the highmagnitude of the received signal strength, when the swimmer is at aclose range, three or four lights may be illuminated at a maximum signalstrength bearing.- Since the magnitude of maximum signal strength isreduced at greater ranges, fewer lights are illuminated when thedirectional receiver is brought into alignment with the beacon. Thus,the swimmer has an approximate idea of the range when he observes thenumber of lights that are turned on when his receiver is pointing at thebeacon.

For a more sophisticated use of the system WhlCh has been described theelectronic equipment associated with the transmitting ultrasonic beacon60 may be provided with coded signals which could be transmitted oncommand. By appropriate additions to the electronic circuit 54, thecoded signals could be used for giving a warning signal to the swimmers.For example, this warning could be used for calling swimmers back Whenthere is an oncoming storm.

A still further use of the system might include means for superimposingspeech signals on the ultrasonic sound waves much as speech is modulatedon a carrier wave in conventional telephony. In this case, a demodulatoris included in the electronic circuitry 54 of FIG. 2. This arrangementyields audible signals which may be transmitted to the swimmer by meansof the underwater earphone 70. If two-way conversation is desired, amicrophone and modulator may also be added to the equipment 54, in whichcase the directional receiver shown in FIG. 2 could be used as anultrasonic speech modulated transmitter in a manner similar to thatdescribed above for the beacon 60.

Although a few specific examples have been chosen to illustrate thebasic principles of the invention, it will be obvious to those skilledin the art that numerous departures may be made from the details shown.Therefore, the invention should not be limited to the specific equipmentshown and described herein. Quite the contrary, the appended claimsshould be construed to cover all equivalents falling within the truespirit of the invention.

I claim:

1. An ultrasonic underwater directional indicator comprising acup-shaped housing structure having a tubular body portion open on oneend and closed on the opposite end, the closed end having a recessedconical frustrum shaped cavity, lined with a layer of 10W acousticimpedance material, means comprising a transducer element, having alongitudinal axis, for transforming ultrasonic sound pressures reachingsaid transducer into corresponding electrical signals, said transducerelement being mounted within said conical frustrum shaped cavity withits longitudinal axis co-linear With the normal axis of said cavity, asound transmitting waterproof compound filling said cavity andencapsulating said transducer element, a Waterproof lid having a rimportion Which mates with the open end portion of said cup-shaped housingstructure, watertight sealing means interposed between said lid and saidopen end of said housing structure whereby a watertight chamber isformed with the combination of said housing and lid assembly, and meansincluding a meter and electrical circuit for giving a positive read-outresponsive to the magnitude of the sonic signal reaching said open end.

2. The invention of claim 1 further characterized in that the ultrasonicfrequency of operation lies in the approximate range 40 kHz. to kHz.

3. An ultrasonic underwater directional indicator comprising acup-shaped housing structure having a tubular body portion open on oneend and closed on the opposite end, the closed end having a recessedconical frustrum shaped cavity, lined with a layer of low acousticimpedance material, means comprising a transducer element, having alongitudinal axis, for transforming ultrasonic sound pressures reachingsaid transducer into corresponding electrical signals, said transducerelement being mounted within said conical frustrum shaped cavity Withits longitudinal axis co-linear with the normal axis of said cavity, asound transmitting waterproof compound filling said cavity andencapsulating said transducer element, a Waterproof lid having a rimportion which mates with the open end portion of said cup-shaped housingstructure, watertight sealing means interposed between said lid and saidopen end of said housing structure whereby a watertight chamber isformed with the combination of said housing and lid assembly, electroniccircuit means assembled within said watertight chamber, said electronic9 a, 7 s circuit means including an ultrasonic amplifier for amp li-References Cited fying the electrical signals generated by saidtransducer UNITED STATES PATENTS element, and means comprising an outputmeter for-indieating the magnitude of said amplified electrical singals.g Z;%Z FTTT 4. The invention of claim? characterized in that said 53O79583 2/1963 Tg 340 5 meter has a face which lies directly below saidwaterproof 3:123:798 3/1964 Holloway et 1 0 lid, and furthercharacterized in that said lid is transpar v 3,2 2,094 7/1966 Camp 340-8X cut to permit an external reading of a scale on the meter.

v 5. The invention of claim v4. and power supply means 10 containedwithin said watertight enclosure for operating said electronic circuitmeans. 340 8 9 RICHARD A. FARLEY, Primary Examiner U.S. c1. X.R. t

